Vaadin Flow Exception on Application call - VaadinSession cannot be cast to VaadinSession - jetty

Question

We're currently developing a new Vaadin Web Application based on a Vaadin Flow Starter pack. It runs with a jetty server, that is started through a maven call. Lately an error has been popping off quite often that prevents me from accessing the application in the browser, resulting in just an exception stacktrace (in the browser as well as in the backend) that says com.vaadin.flow.server.VaadinSession cannot be cast to com.vaadin.flow.server.VaadinSession. Here's the full stacktrace.
javax.servlet.ServletException: javax.servlet.ServletException: com.vaadin.flow.server.ServiceException: java.lang.ClassCastException: com.vaadin.flow.server.VaadinSession cannot be cast to com.vaadin.flow.server.VaadinSession
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:146)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.Server.handle(Server.java:531)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:352)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:260)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:281)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:102)
at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:118)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:333)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:310)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:168)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:126)
at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:366)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:762)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:680)
at java.lang.Thread.run(Thread.java:748)
Caused by: javax.servlet.ServletException: com.vaadin.flow.server.ServiceException: java.lang.ClassCastException: com.vaadin.flow.server.VaadinSession cannot be cast to com.vaadin.flow.server.VaadinSession
at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:302)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:857)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:535)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:146)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:548)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:257)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1595)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:255)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1317)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:203)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:473)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1564)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:201)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1219)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:144)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:219)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:126)
... 15 more
Caused by: com.vaadin.flow.server.ServiceException: java.lang.ClassCastException: com.vaadin.flow.server.VaadinSession cannot be cast to com.vaadin.flow.server.VaadinSession
at com.vaadin.flow.server.VaadinService.handleExceptionDuringRequest(VaadinService.java:1535)
at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1498)
at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:300)
... 33 more
Caused by: java.lang.ClassCastException: com.vaadin.flow.server.VaadinSession cannot be cast to com.vaadin.flow.server.VaadinSession
at com.vaadin.flow.server.VaadinService.readFromHttpSession(VaadinService.java:2071)
at com.vaadin.flow.server.VaadinService.loadSession(VaadinService.java:2051)
at com.vaadin.flow.server.VaadinService.getExistingSession(VaadinService.java:942)
at com.vaadin.flow.server.VaadinService.doFindOrCreateVaadinSession(VaadinService.java:791)
at com.vaadin.flow.server.VaadinService.findOrCreateVaadinSession(VaadinService.java:766)
at com.vaadin.flow.server.VaadinService.findVaadinSession(VaadinService.java:624)
at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1480)
... 34 more
Now the stacktrace itself is one of a kind and I have not seen something like it before, because it says it can't cast the VaadinSession to a VaadinSession, which is unfortunate. I'm pretty sure this stuff is handled from Vaadin itself and I'm just using it as a blackbox, so right now I don't really know where this errors comes from and how I can fix it.
I am extending a VaadinServlet to hook into servlet and session initialization, the code is pretty straight forward though (although I have to say that I'm not a master in session initaliziation, so I just ripped it off some other tutorial). I'm not sure if this may have something to do with it, here's the code anyways.
#Override
protected void servletInitialized() throws ServletException {
super.servletInitialized();
getService().addSessionInitListener(this);
getService().addSessionDestroyListener(this);
logger.info("servlet initialized");
}
#Override
public void sessionInit(SessionInitEvent event) throws ServiceException {
// Do session start stuff here
logger.info("session initialized");
SessionBean sessionBean = new SessionBean();
sessionBean.setPlant(Config.getInstance().getPlant());
// TODO Berechtigungen aus dem Nutzer auslesen
sessionBean.setAuthorizations(AUTHS);
event.getSession().setAttribute(SessionBean.class, sessionBean);
event.getSession().setAttribute(BPService.class, new BPService());
}
#Override
public void sessionDestroy(SessionDestroyEvent event) {
event.getSession().setAttribute(SessionBean.class, null);
event.getSession().setAttribute(BPService.class, null);
}
The error occurs on hot deploys as well as fresh starts. A restart of the application server does not necessarily resolve the problem. As soon as the error occurs in a page call (does not matter which browser, I had these errors in Chrome, Opera, Firefox and IE) it is repeated over and over again if I refresh this particular page or try to call it from a new window of the same browser. If I switch to an incognito window or switch the browser, it will not occur in the new window, but still keeps popping off in the old one.
Current configuration
Vaadin 12.0.0 (happened with 11.0.2 too)
mvn jetty:run with a jetty 9.4.11.v20180605
java 1.8
I think I'm currently using the full Vaadin Flow stuff, I'm not sure how it handels the dependencies, but this is my only dependecy for Vaadin right now:
<dependency>
<groupId>com.vaadin</groupId>
<artifactId>vaadin-bom</artifactId>
<version>${vaadin.version}</version>
<type>pom</type>
<scope>import</scope>
</dependency>
I tried switching the jetty server to a different version with no success. Updating Vaadin to 12 from 11 also had no impact on the error.